chore: add README contents as pypi long_description

The `pypi-publish` action runs `twine check` which complains that this
field is missing.

Author: ahal

setup.py

@@ -13,10 +13,14 @@
 with open(os.path.join(project_dir, "requirements/base.in")) as fp:
     requirements = fp.read().splitlines()
 
+with open(os.path.join(project_dir, "README.rst")) as fh:
+    long_description = fh.read()
+
 setup(
     name="taskcluster-taskgraph",
     version=namespace["__version__"],
     description="Build taskcluster taskgraphs",
+    long_description=long_description,
     url="https://github.com/taskcluster/taskgraph",
     packages=find_packages("src"),
     package_dir={"": "src"},

taskcluster/docker/decision/Dockerfile

@@ -11,6 +11,8 @@ RUN mkdir /builds && \
 ADD topsrcdir/src/taskgraph /setup/taskgraph/src/taskgraph
 # %include setup.py
 ADD topsrcdir/setup.py /setup/taskgraph/setup.py
+# %include README.rst
+ADD topsrcdir/README.rst /setup/taskgraph/README.rst
 # %include requirements/base.in
 ADD topsrcdir/requirements/base.in /setup/taskgraph/requirements/base.in
 # %include requirements/base.txt